An optical time-domain reflectometer (OTDR) is an optoelectronic instrument and can be used to characterize an optical fiber. In optical time-domain reflectometry short optical pulses are transmitted into an optical fiber and the backscattered power can be measured as a function of time. From the recorded time trace a spatial distribution of backscattering and attenuation can be derived. As the backscattered power P is relatively low and distributed over time an optical time-domain reflectometer requires a sensitive receiver. For this purpose usually an avalanche photo diode (APD) is used as a receiving element of the optical time-domain reflectometer. The strength of the returned optical pulses can be measured and integrated as a function of time. When a strong optical signal is received resulting from a high reflection point along the optical fiber under test FLUT the photo diode of the optical time-domain reflectometer can become saturated and is then not able to detect further optical signals for some time. Further any lower reflection close to the high reflection point cannot be detected.
FIG. 1 shows a conventional setup of an optical time-domain reflectometer. The OTDR shown in FIG. 1 comprises a laser which can generate a short optical pulse which is transmitted into the optical fiber under test FLUT. The generated short optical pulse generated by the laser can be sent via an optical circulator and a wavelength division multiplexer (WDM) into the optical fiber under test. The optical pulse propagates in the optical fiber under test and a fraction of its power is constantly scattered. This results in a loss of the pulse power. The scattered power captured by the optical fiber propagates in reverse direction towards the wavelength division multiplexer WDM. At the wavelength division multiplexer WDM the scattered optical power is directed via the circulator of the optical time-domain reflectometer OTDR apparatus towards a photo diode of the OTDR apparatus. The photo diode recalls the time evolution of the received backscattered power, from which a location of larger reflections can be derived as well as the attenuation of the power along the fiber link under test FLUT. When a strong optical signal from a high reflection point is detected by the photo diode the photo diode can become saturated and is no longer able to detect further optical signal for some time. Typically a high reflection point can be located at the launch point of the optical pulse, i.e. at the wavelength division multiplexer, but high reflection points can be located at any point along the optical fiber under test FLUT.
FIG. 2 shows a typical backscattered trace of a fiber link under test FLUT where a broad pulse at the launch point indicates saturation of the photo diode and the corresponding resulting dead zone. As also shown in FIG. 2 there is a strong reflectance of a connector, likewise leading to a dead zone in the backscattered trace of the fiber link under test FLUT. Accordingly an OTDR dead zone DZ refers to a distance (or time) where the optical time-domain reflectometer cannot detect or precisely localize any event or artefact on the fiber link under test FLUT. An OTDR dead zone DZ is always prominent at the beginning of a trace or at any other high reflection point.
Accordingly there is a need to provide a method and apparatus for measurement of a backscattered trace of a fiber link under test where quality and precision of the measurement is increased.